Since the dawn of the industrial age, engineers have searched for more efficient and cost effective methods and means for manufacturing. Automating the assembly line with robotic machines faithfully repeating the same motions with a high level of accuracy and speed has significantly improved the efficiency of manufacturing. This automation leads to concomitant decrease in costs while providing improved quality control. However, the more sophisticated the automation, the greater the need for more sophisticated control systems to manage the robots.
Many tasks in manufacture and assembly involve the joining of two or more parts. These tasks, and many others, are difficult for a robotic machine to repeatedly perform well. Examples are seam welding or gluing, or more collectively, any task that requires the robotic machine to follow a path on the workpiece, such as the seam. For many workpieces, this seam meanders in three dimensions. The fundamental difficulty for the robotic machine is the geometric problem of positioning the tool, then moving the tool to maintain close position tolerance. The positioning step must be done with the correct translational and rotational orientation relative to a curved seam lying in three dimensional space. The robotic machine must then be able to follow the seam while maintaining the proper orientation and traveling at relatively high speeds. In the absence of some probe or other seam tracking device, the robot must be manually taught to follow a seam. Manually teaching a robot each and every minute maneuver is a time consuming, labor intensive task.
A complication is that no two workpieces are the same. Workpieces vary because as the various individual parts to be assembled are manufactured, the dies, cutters, extruders and stamps used to produce the individual pieces are subject to wear and changing environmental conditions. Changes in the die, cutters, material properties, etc., cause changes in the overall dimensions and quality of any given piece compared to its design standard. A manually taught robot must then be programmed to account for the variation from one workpiece to the next. However, this reprogramming requires an operator to anticipate which way the variation will occur, a task that is not easily accomplished.
One of the advantages of using robotic machines in manufacture and assembly is that the machines may be taught a particular task that the robot will repeat nearly flawlessly and with substantially higher speed than more conventional, or traditional, means of accomplishing the task. However, how the robotic machine is taught the task usually requires considerable forethought and design engineering.
For example, with lap joint welding, the task varies from simple to complex depending on the complexity of the joint. If the joint is formed between two flat, straight pieces of metal, the task for the robotic machine is simple. The machine is taught to align the welding gun at a certain orientation to the lap joint and move along the joint confined to a single direction. Such a process is easy to reproduce and is reliable, but also requires that either the workpieces or must be fed in from the same direction and with the same orientation each time or the robotic arm limited to moving in the same direction and orientation. This type of automatic system is useful, but is limited, because the entire assembly plant must be planned in a specific fashion to feed the pieces to the robotic machine in a particular way in order to accommodate the limitations imposed on the robotic machine by the closed loop control architecture.
Design engineering has led to the development of industrial robots with open control architecture. These robots have six degrees of freedom, those being translation along the X, Y, and Z axes, and rotation, or orientation, about the X, Y, and Z axes. A robot with open control architecture is able to integrate with a seam tracking device to provide the robot the ability to teach itself the path that it is to follow. How the robot performs this is dependent on the type of seam tracking device used and how the seam tracking device interfaces with the robot control architecture. An example of a seam tracking device is a tactile probe.
An example of an early closed loop control architecture robot using a two dimensional tactile probe is described in Automated Welding Using Special Seam Tracing; Bollinger, John G. and Harrison Howard L., Welding Journal, pages 787-792, November 1971. In this paper, the authors describe the basic geometry and coordinate definitions of a five axis tracing machine using a two dimensional probe.
The tactile probe carried at the distal end of the robot arm traditionally has also been restricted in axial degrees of freedom, usually just to the X and Z axes, with X defined as being lateral to the instantaneous direction of travel, Y being along the path, or tangential to the path, and Z being normal to surface of the path. With two dimensional tracing probes, it is necessary to constrain the device in some fashion in order to maintain a required orientation to the seam or path so that the mechanism measuring the X axis remains orthogonal to the path. This requirement places an additional control burden on the robot arm and the control systems.
Consequently, there is a need for a three dimensional tactile probe, integrated with a robotic arm machine having an open control architecture, for tracing paths and seams along a workpiece that is not encumbered by restrictions in orientation of the tactile probe or tactile probe sensors to the workpiece path or seam while following the path or seam in three dimensions.